Not Applicable
1. Field of the Invention
The present invention relates to an apparatus for and a method of making very accurate non-invasive measurements of electrical current flowing from a power source to a device under test (DUT).
2. Description of the Related Art
In testing of electronic devices, voltage and current waveforms are often measured during various modes of operation of the electronic device as an indicator of proper device functionality. In order to make waveform measurements, the power source which is used for supplying power to the electronic device under test (DUT) must be separated from the DUT to allow for addition of measurement instruments. Referring to FIG. 1, ideally, power is supplied from a voltage source 103 to a device under test 102 and no additional series impedance is encountered between the voltage source 103 and the device under test 102. Therefore, the open circuit voltage at the DUT is equal to VS and the DC source impedance of the voltage source 103 is equal to RS. In order to make the necessary current measurements a measurement system and wiring are added to the circuit, which introduces non-desirable impedances.
In a conventional measurement system shown in FIG. 2, a device under test 102 is supplied from a power source 103 through wire impedances which are indicated as lumped parameters ZA and ZC. A current is sensed by a non-contact measuring probe 104, such as for example, a current transformer or a hall effect sensor, and a representation of the current is displayed on an oscilloscope 105. However, in such a measuring system, a transient voltage dip and overshoot results due to the additional insertion impedances of the probe, as well as from ZA and ZC. This voltage transient can overvoltage or undervoltage the DUT, potentially terminating the intended test or damaging the DUT.
Another conventional measuring system is shown in FIG. 3. A current measurement shunt RM is inserted in series with the voltage source 103 and the device under test 102. An additional voltage drop will be realized due to the measurement shunt RM and the wire impedances, represented by the lumped parameters ZA, ZB and ZC.
Yet another conventional measuring system is shown in FIG. 4. Using a dc power supply 106 with conventional two-wire sense, in a pass through mode where the supply is set to zero volts, current from the voltage source 103 can be measured. By using inputs sense (xe2x88x92) and sense (+), the power supply 106 compensates for voltage drops across the measurement system 106 and the impedances ZA and ZB but not for a voltage drop across the impedance ZC.
In the present invention, a second set of sense leads is provided for the power supply. This allows the supply to compensate for the additional voltage drop across impedance ZC within the bandwidth limitations of the power supply. By setting the power supply to zero volts a total of the errors introduced by the measurement system, ZA, ZB, and ZC is reduced nearly to zero within the bandwidth limits of the power supply. As a result, the open circuit voltage and source impedance at the DUT equals VS and RS respectively. Thus the required current measurements may be taken in a non-invasive manner.